To understand how data travels, we use layered models. Each layer has a precise job and talks to the layers directly above and below it.

The OSI model (7 layers)

• 7 Application — network services (HTTP, DNS).
• 6 Presentation — formatting and encryption.
• 5 Session — opening and tracking conversations.
• 4 Transport — TCP and UDP, ports.
• 3 Network — IP addressing and routing.
• 2 Data link — MAC addresses, switching.
• 1 Physical — cables and signals.

The TCP/IP model is more compact (4 layers: Application, Transport, Internet, Network access) and matches what is actually implemented on the Internet. Data is encapsulated: each layer adds its own header (segment, packet, frame).

Data     -> Application layer
Segment  -> Transport layer (TCP/UDP)
Packet   -> Internet layer (IP)
Frame    -> Network access layer (MAC)
Bits     -> Physical medium

Knowing these layers helps you place a fault: an unplugged cable is a layer 1 problem, a wrong IP address a layer 3 one.

Key point: OSI has 7 layers, TCP/IP has 4. Encapsulation adds a header at every layer on the way down. See Lab 2 – Observing encapsulation in Packet Tracer.